In a yarn brake on a yarn feeding device as known from PCT/EP 94/00476, the braking band is glued into the small diameter end portion of a generally conical rubber membrane circumscribing the storage body and being supported at its large diameter end in the stationary support to pull the braking band inner side against the withdrawal rim of the storage body. Such yarn brakes are extensively used at yarn feeding devices for projectile and rapier weaving machines. The membrane commonly constitutes the braking band carrier, and the spring element transmits the axial pressing force onto the braking band without disturbing the local radial deformability of the braking band. Even though it is intended to make the membrane as homogenous as possible nevertheless in operation of the yarn brake the pressing force of the braking band against the withdrawal rim may vary in the circumferential direction. This causes undesirable fluctuation of the withdrawal tension level of the yarn pulled through between the inner side of the braking band and the withdrawal rim and is simultaneously orbiting in the circumferential direction. Gluing the braking band to the braking band carrier is time-consuming and costly. The junction region creates an inhomogeneity which negatively influences a uniform distribution of the pressing force of the brake band along the braking zone. In addition, the friction of the yarn orbiting during withdrawal is transmitted by the braking band into the membrane, which generates a torsional tension in the membrane as the membrane is firmly secured in the large diameter end portion in the support. Said effect caused by the yarn may additionally complicate the uniformity of pressing the braking band against the withdrawal rim. The known yarn brake, however, fulfills a particularly desirable self-compensating effect since it automatically decreases its braking effect with increasing or high withdrawal speed of the yarn and an increase in the yarn tension or a high yarn tension level caused by said withdrawal speed, but the circumferentially varying pressing force of the braking band is disturbing, particularly in the case of sensitive yarn qualities and in modern high performance weaving machines operating with high insertion frequency and extreme insertion speed variations.
A self-regulating yarn brake having a radially deformable braking band with the shape of a frustocone for the first time has been disclosed in EP-A-0 534 263. Said braking band is firmly glued to the inner side of a frustoconical braking band carrier which is radially deformable but axially stiff and is axially loaded in the stationary support by means of a spring assembly. The gluing junction area between the braking band and the braking band carrier has to be made precisely and uniformly and may influence the uniform pressing force distribution of the braking band.
In order to replace the braking band in such yarn brakes having a durable, firm connection between the braking band and the braking band carrier, the braking band carrier has to be disassembled as well and has to be discarded.
It is an object of the invention to create a yarn feeding device of the kind as disclosed as well as a yarn brake for such a yarn feeding device wherein the important advantage of the self-compensating effect is maintained, a negative influencing of the yarn withdrawal tension level due to a circumferentially varying pressing force can be avoided, and wherein the yarn brake is easy to manufacture and handle.
This object can be achieved by supporting the braking band in a floating manner relative to the braking band carrier and the withdrawal rim.
Due to the holding of the braking band in a floating manner, a labour and cost intensive gluing connection between the braking band and the braking band carrier is avoided. In connection therewith the almost unavoidable inhomogeneity of a gluing connection in the force transmission is avoided. The braking band is pressed against the withdrawal rim circumferentially uniformly and regularly. The torsional force generated by the orbiting motion of the yarn and the friction contact of the yarn with the inner side of the braking band is not or not significantly transmitted into the braking band carrier such that the negative influence of such torsional force on the contact between the braking band and the withdrawal rim is avoided. The braking band may move in relation to the braking band carrier during operation and could even rotate. A floating holding of the braking band means a positive engagement which hinders losing the braking band in case of missed contact with the withdrawal rim. However, there is no structural connection between both components but only some sort of a loose trapping. This contributes to the uniformity of the contact pressure. In case that the braking has to be replaced, e.g. due to wear or for adaptation to particular yarn qualities, this can be made easily. The braking band carrier can be used longer and does not need to be disassembled. In order to provide an optimum self-centering of the braking band on the withdrawal rim it can be expedient to support the braking band carrier in a self-centering Cardanic fashion.
The yarn brake can be manufactured with fair costs and in a simple fashion; mounting and maintenance are easy to carry out. The pressing contact relationships between the braking band and the withdrawal rim are optimum thanks to the floating holding, since there is no structural connection between the braking band carrier and the braking band which causes a disturbance when transmitting the axial pre-load force.
The braking band is apt to position itself at least in tangential directions and relative to the normally rounded withdrawal rim, i.e. the braking band is apt to rotate about the axis of the storage body and to displace itself also in the direction of the generatrix of the frustocone or, respectively, to tilt in the braking zone without the necessity that the braking band carrier follow such movements or suppress such movements.
In contradiction to the conventional principle of a circumferentially uniform contact between the braking band carrier and the braking band, the advantage of the floating holding of the braking band is optimized. In the circumferential direction suspension ranges axially pressing against the braking band alternate with ranges not pressing or only pressing with reduced extent. Due to its elastic behaviour the braking is compensating for such differences in the pressing behaviour in an effective manner, particularly with higher yarn speeds. Occasionally the yarn may feel differences in the pressing force of the braking band in the braking zone with low yarn speed. However, this leads to the advantage of relatively high basic yarn tension in the yarn at low yarn speeds and under the axial pre-load of the braking band, which pre-load occasionally and nominally can be lower than in case of a full circumferential pressing relation. Thanks to the self-compensating effect fulfilled by the floating or suspended braking band the difference of the yarn tension between slow and highest yarn speeds remains desirably small, since the braking band does reduce the braking effect automatically with increasing yarn speed. Such a reduced axial pre-load nevertheless leads to a sufficiently high basic yarn tension and relieves the components of the yarn brake and improves the durability.
In one embodiment, the braking band carrier includes a plurality of circumferentially distributed suspension ranges separated by interspaces which contribute to a regular varying braking effect (digital plucking) and to a high basic yarn tension level already with moderate axial pre-load.
The suspension ranges may be formed resiliently in order to influence the local deformability of the braking band during the passage of the yarn circulating in circumferential direction as little as possible.
Further, the suspension ranges may be arranged at arms of the braking band carrier. Additionally or alternatively said arms may be resilient. This allows a gradual adjustment of the axial preload adapted to the initial operating conditions.
The dimensional extension of the braking band in the direction of the frustocone generatrix thereof is larger than the dimensional extension of each suspension range in the same direction, the dimensional extension of the suspension range in the above direction is larger than the width of the braking zone, and each suspension range extends substantially symmetrically beyond the extension of said braking zone. The braking band carrier thus actuates the braking band in the braking zone and even also in the neighbourhood thereof.
A braking band holding means is provided and grips outwardly and/or inwardly around at least one of the braking band edges with a clearance. The handling is improved since the braking band normally positioned by the axial contact pressure is prevented from being lost even when the axial pre-load force is relieved (e.g. for threading the yarn, during the mounting procedure or during the transport of the yarn brake). The holding of the braking band is intended to allow a comfortable mounting and also a quick replacement of the braking band and should only assure the necessary safety against loosing the braking band.
A properly controlled entrance of the yarn into the yarn brake is important for an optimum braking effect and for the self-compensating effect. In addition, it has to be assured in each rest period in-between insertion cycles that the yarn brought to a standstill will not be caught at the yarn brake. This object is solved by providing a yarn guiding element which also serves as an outer braking band holding means and which can be secured together with the braking band carrier in the support. Apparently, the yarn guiding element could also be suspended separately from the yarn brake. In such a case it can be preferable to additionally provide a braking band holding means at or in the braking band carrier.
In one embodiment, the braking band carrier at least in the axial direction has spring-like properties, and the axial pre-load of the braking band against the withdrawal rim is adjusted by the axial position of the support relative to the withdrawal rim.
The braking band carrier within the support or together with the support is axially loaded by an adjustable spring assembly. The axial pre-load of the braking band against the withdrawal rim can accordingly be adjusted very sensitively, e.g. when using a spring assembly having a relatively long operating stroke and advantageous spring characteristics.
The braking band is suspended inside of a frustocone jacket in a floating non-loosening fashion. The frustocone jacket forms the braking band carrier. Said frustocone jacket could be made, e.g. from plastics reinforced by carbon fibres and is axially stiff, radially deformable and lightweight.
A wearing element, e.g. with the shape of a frustocone jacket, is inserted between the braking band and the braking band carrier. Said wearing element does influence the local deformability of the braking band as little as possible but is distributing or minimizing wear between the braking band and the braking band carrier. Said wearing element could be made from easy gliding material in order to minimize the motion or float resistance of the braking band relative to the braking band carrier.
The arms of the braking band carrier are inwardly protruding spokes, spring lamellas or pins which are provided at an annular base body in radial planes about the axis. Equal pressing forces are achieved in the circumferentially spaced apart suspension ranges. The spring behaviour can be optimally predetermined by the design of the arms and their material choice. The braking band carrier is lightweight and is characterized by low mass, particularly in the contact region with the braking band. With the arms situated essentially in radial planes about the axis of the storage drum, offset reaction forces of the arms are avoided when the braking band is deformed.
Alternatively, said arms can be oriented obliquely in relation to radiuses on the axis, e.g. in order to increase the useable a arm length or to reduce the interspaces between the suspension ranges or to achieve in the circumferential direction an overlap or superimposing between the discrete suspension ranges.
The braking band carrier can be embodied by a circumferentially continuous ring which carries a circular spiral spring, the spring windings of which define the suspension ranges. The axial pre-load from the circular spiral spring is transmitted onto the braking band. Within very little space many uniformly acting suspension ranges are achieved.
The spring windings are obliquely angled in relation to the spring axis, and a soft response behaviour in said suspension ranges is achieved by the oblique orientation of the spring windings.
Elastically deformable braking band carrier variants are provided, such as a frustoconical annular membrane having at least one concentric undulation, a deformable ring chamber containing a fluid filling, or a resilient ring body made from elastic material such as rubber or foam, wherein thanks to the floating holding disturbing gluing junctions are avoided which contributes to the circumferential uniformity of the contact pressure. By means of said braking band carrier variants the axial pre-load can be adjusted very sensitively and gradually and compact yarn brakes can be achieved.
The braking band carrier transmits the axial pre-load either via a suspension zone which is circumferentially continuous and smooth or via a suspension zone which has separated hills and valleys. In each case the braking band is held floatingly.
The braking band is held in a pre-shaped pocket or within a plurality of circumferentially distributed pockets formed in the braking band carrier such that it cannot be lost. The pocket or pockets are pre-shaped in the braking band carrier, e.g. an elastic annular membrane made of polyurethane. This design is structurally simple and allows a simple mounting. The braking band and the braking band carrier are apt to move in relation to each other in case of forces caused by the operation. In each case a safe force transmission from the braking band carrier into the brake band and vice versa is assured.
Alternatively, several ears defined by slots in the braking band carrier hold the braking band. The outer edge of the braking band is inserted below said ears. Advantageously a simple tool similar to the horn of an anvil is used for mounting the braking band in the two last mentioned embodiments of the braking band holding. Said tool allows an easy opening of the pocket or the pockets to introduce the outer edge of the braking band.